Conventionally, in a radar apparatus for detecting objects in all directions from one's own vessel, a reception signal of a polar coordinate system is obtained while rotating a radar antenna at a predetermined period. The radar device converts a reception signal of this polar coordinate system to image data of a rectangular coordinate system and writes the image data to an image memory, and at a predetermined timing reads out image data that has been stored in the image memory. In a display of the radar apparatus, display is performed with the emission intensity and color changed according to the data level of the read-out image data.
In this sort of radar apparatus, there is a display method in which during one revolution (referred to as one sweep rotation) of the radar antenna, the emission intensity or the like of the displayed data is gradually weakened.
For example, in JP 2000-65920A, a radar device is disclosed in which image data stored in an image memory is read out, and the data level is gradually reduced by subtracting from the data level by predetermined amounts at a time according to a subtraction period, and the image data is stored in a successively updated manner in the image memory.
Also, in JP 2004-354301A, a radar device is disclosed in which instead of the subtraction process indicated in JP 2000-65920A, the data level is reduced by multiplying by a coefficient that is less than one.
In conventional radar devices such as those described in JP 2000-65920A and JP 2004-354301A, the aforementioned display method in which the emission intensity is gradually weakened during one sweep rotation is used in order to more easily distinguish actually present fixed or moving objects from white noise, interference from a radar of another vessel, or rain.
However, in these conventional radar devices, with respect to devices that have the same data level as each other, their emission intensity weakens approximately the same amount. Therefore, for example, a high data level is temporarily detected due to radar interference, and if the data level is the same as that of an actual object, it is not possible to clearly distinguish the radar interference and the actual object. Also, when, for example, gain has been raised in order to display low level data of an object that is far away at a high brightness, the far away object certainly can be displayed at a high brightness, but the amount of white noise having about the same level also increases, so it becomes difficult to distinguish the object from the white noise.